Side-by-side comparison
| Parameter | Induction Motor | Synchronous Motor |
|---|---|---|
| Speed | N = Ns(1 – s); always less than synchronous speed (slip 2–5%) | N = Ns = 120f/P exactly; runs at synchronous speed always |
| Slip | Non-zero — slip creates rotor current and torque | Zero at steady state — rotor locked to field |
| Self-Starting | Yes — starts and accelerates to near-synchronous speed | No — needs damper windings or auxiliary motor to start |
| Power Factor | Always lagging (0.8–0.9 lagging typical); cannot be varied | Adjustable: leading, unity, or lagging by varying DC excitation |
| DC Excitation | Not required | Required for rotor field (separately excited with DC) |
| Hunting | Does not hunt — no synchronizing torque | Hunts if load suddenly changes; damper windings reduce hunting |
| Efficiency | 92–96% at full load for large motors | Slightly higher at large ratings; no slip loss |
| Application | Pumps, fans, compressors, conveyors — general industrial drives | Power factor correction, constant-speed loads: synchronous condensers, large compressors above 1 MW |
Key differences
Induction motors slip behind synchronous speed — a 4-pole, 50 Hz motor has Ns = 1500 rpm but runs at 1440 rpm under full load (slip = 4%). That slip is essential: without it, no current would be induced in the rotor and no torque would develop. Synchronous motors run at exactly Ns = 120f/P — a 6-pole, 50 Hz synchronous motor locks at 1000 rpm. They cannot start on their own (rotor has no induced currents at standstill) and need damper windings for run-up. Over-excitation makes synchronous motors draw leading current, effectively acting as capacitor banks — synchronous condensers in substations exploit this for reactive power compensation.
When to use Induction Motor
Use an induction motor for any self-starting, variable-load industrial application — for example, a 75 kW, 415 V, 3-phase squirrel cage induction motor on a centrifugal pump in a chemical plant.
When to use Synchronous Motor
Use a synchronous motor when you need exactly constant speed, power factor correction, or both — for example, a 2 MW, 11 kV synchronous motor driving a large air compressor in a steel plant while simultaneously correcting the plant power factor from 0.75 to 0.95 leading.
Recommendation
For exam problems, choose induction motor when self-starting is required and power factor correction is not mentioned. Choose synchronous motor when the question mentions leading power factor, synchronous condenser, or exact speed. That pair of conditions resolves most motor-selection exam questions.
Exam tip: Examiners ask why synchronous motors cannot start on their own — explain that at standstill the rotor cannot follow the rapidly rotating stator field (50 Hz = 3000 rpm for 2-pole), so no net torque develops; damper windings provide asynchronous starting torque.
Interview tip: Interviewers at power plants ask about V-curves of synchronous motors — explain that the graph of armature current vs. field current is V-shaped, with unity power factor at the minimum current point and leading/lagging on either side.